Not Applicable
Not Applicable
The invention relates to a driving system for industrial trucks.
Combustion engine driven industrial trucks require a transmission to provide the desired torque at the desired speed. It is known to use a change-speed mechanism including a clutch for this purpose. However, such a system is unacceptable for industrial trucks. Therefore, it is also known in driving systems to provide hydrodynamic transducers in combination with single-step transmissions and multi-step transmissions. This solution proves disadvantageous at slow driving speeds because efficiency is not good in this operating condition. Furthermore, a clutch function is required to independently operate the lifting function.
The disadvantages of the hydrodynamic solution are not encountered in a hydrostatic driving branch. However, what opposes the relatively high efficiency when driving at a slow speed is the bad efficiency at faster driving speeds. Another drawback is the relatively large construction expenditure due to the high hydraulic pressures up to 400 bar and the system""s susceptibility to defects, particularly with regard to leakage.
Finally, in the industrial truck field, it is also known to perform the transmission of forces by means of an electric transmission. In an electric transmission, all the energy generated by the combustion engine is converted into electric energy by means of a generator. Then, this electric energy is reconverted into mechanical energy by means of an electric motor. The drive controls for electric motors are capable of adjusting any gear ratios between the generator and the electric motor. Hence, a system of this type has the advantage of being regulable particularly well. A drawback, however, are the low efficiencies due to the high losses in the controls and electric machines.
The existing solutions in driving combustion engine operated industrial trucks, as a rule, involve considerable energy losses in certain operating situations. Furthermore, the existing systems are incapable of returning energy in braking. Furthermore, emissionless operation is impossible, which nearly rules out the use of such industrial trucks in closed rooms. Another disadvantage of the former driving systems lies in the fact that the components of the systems require to be designed for peak loads. The mean load acting on the components, however, clearly is below such values. Thus, a large construction expenditure is incurred that is not needed for the mean.
DE 198 03 160 has made known a two-branch driving system for automobiles which is intended to better meet the requirements to power-driven vehicles in a stop-and-go operation in conurbations. The known system is a so-called hybrid drive with a first electric machine disposed on a driving shaft of the hybrid drive and a second electric machine the two of which are always operable as a generator and an electric motor and which are coupled to each other and to the combustion engine via a two-branch gear transmission. The second electric machine is positioned on a shaft with which a rotary brake is associated which is controllable in dependence on the driving condition of the power-driven vehicle and by which the respective shaft can be located. It is by means of such a driving system that a more favourable efficiency behaviour of power-driven vehicles and a long service life of the drive are intended to be ensured.
DE 197 47 459 has made known a two-branch driving system for tractors and agricultural machinery which has a mechanical drive and an adjustable hydrostatic transmission. This driving system is intended to increase efficiency via the mechanical branch. In addition, a driving system of this type may obtain a higher speed as compared to simple hydrostatic driving systems. The known driving system, as a rule, does not need the same forward movement and reverse movement speed as is the case in industrial trucks. The earlier described hydrid driving system does not make it possible either to reverse at a maximum speed.
It is the object of the invention to provide a driving system for industrial trucks that enables a large range of speed adjustability at a uniformly good efficiency and does not limit the manoeuvrability of the vehicle. In addition, one embodiment is also intended to make possible emissionless operation in closed rooms at least temporarily.
In the inventive driving system for industrial trucks, a two-branch system is provided wherein the primary branch provides a mechanical no-clutch connection between the combustion engine and the driven shaft whereas an adjustable transmission is disposed with no clutch in a secondary branch. The primary and secondary shafts are at a fixed speed ratio to each other with the gear ratio of the two-branch gear transmission being 1, for example. In a gear assembly, the speed of the primary and secondary shafts are summed up. To realize a zero number of revolutions on the driven shaft (with the vehicle stopped) the primary branch and the secondary branch, as a rule, need to have a counter-rotation sense. Therefore, a reversal of the sense of rotation requires to occur in the primary or secondary branch. This one may be effected either in the two-branch gear transmission, the adjustable transmission or the gear assembly. The output shaft of the gear assembly is coupled to a change-over transmission in order to selectively reverse the sense of rotation of the driven shaft of the driving system and, hence, the direction of travel.
The adjustable transmission can be designed differently and be, for example, an adjustable electric, hydrostatic, hydrodynamic, pneumatic or mechanical transmission. As adjustable mechanical transmissions, various infinitely variable transmissions are imaginable such as a V-belt driven variable transmission, a flat-belt driven variable transmission, a steel thrust belt driven variable transmission, a steel thrust chain variable transmission or a friction gear variable transmission. The gear ratio of the adjustable transmission may be infinitely varied when in operation. Since the gear assembly sums up the speeds of the shafts the speed of the driving system""s driven shaft may also be varied from zero to the maximum speed. For the rest, the gear assembly may be put in the place of the two-branch gear transmission, and vice versa, in which case, however, no adjustable mechanical transmission can be provided. Various variants are imaginable to realize the gear assembly. In an aspect, it may be a planetary mechanism or a differential mechanism.
In another aspect of the invention, an accumulator may be associated with the adjustable transmission. In a hydrostatic transmission, a pressure accumulator will have to be provided. In an electric transmission, this accumulator is a battery. If an accumulator is used the vehicle may be operated without any exhaust gases in order to enable it, for example, to be driven in closed rooms. Moreover, energy recovery may be possible during braking.
If the vehicle has stopped and the combustion engine has been started this one will rotate at an idle speed. At a gear ratio of 1, the primary shaft and the secondary shaft will rotate at the same speed, but in opposite senses. If the adjustable transmission is adjusted so as to rotate the driven shaft at the same speed as the primary shaft there will be no speed on the driven shaft of the driving system. The vehicle will stand at idle. To make it start up the gear ratio is changed.
The inventive driving system involves a series of advantages. The speed variation range will result to be larger than that of single-branch driving systems. A high efficiency will be obtained because some part of the power is transmitted in a directly mechanical way. The adjustable transmission which is relatively expensive need not be designed for a maximum power. Such a driving system has several advantages:
Transmissions having a variable gear ratio generally are of an efficiency which is lower than the one of those having a constant gear ratio. Therefore, the power loss decreases and the overall efficiency of the drive increases.
Because of the lower demands to power, adjustable drives may be used the capacity of which would not be sufficient otherwise, e.g. V-belt driven variable transmissions. In addition, the adjustable drive may be dimensioned smaller.
The gear ratio between the combustion engine and the output shaft of the driving system may be made to be zero. This enables the vehicle to be kept at stoppage while the combustion engine is running with the flux of power not being interrupted by an engaging and disengaging clutch. In addition, the spread of the gear ratio takes on any magnitude desired in this manner. (The spread referred to here is known to be the relationship between the largest and the smallest gear ratio of an adjustable transmission).
However, a driving system possesses the two first advantages only if a change-over transmission is provided for the reversal of the sense of rotation when a change is made between the forward and backward travels. It is true that the reversal of the sense of rotation could even be realized without any change-over transmission, but only by varying the gear ratio of the adjustable transmission. Without any change-over transmission, the maximum power to be transmitted by the adjustable transmission at a backward travel at a maximum speed would be larger than the output power of the driving system""s driving shaft.
If an accumulator is provided the combustion engine may be designed to be smaller for the permanent output power produced. Furthermore, an accumulator permits to drive with no exhaust gas. Energy may be recovered during braking. This reduces the consumption of energy and the requirements to the thermal load-carrying capacity of the operating brakes will be lower.